Atargatis and Ichthys
Atargatis (pronounced ay-tar-GAY-tis) was an ancient Syrian goddess. Her son was Ichthys (pronounced ICK-this), whose symbol was a fish. Today that symbol is very well-known. An ancient writer speaks of a statue of Atargatis with the usual mermaid appearance, but surviving relics show her as a fish with a woman's head and legs. Her cult was spread from Syria by sea-going merchants to Greece and Italy and by slaves to the far north of the Roman Empire. Perhaps this is why there are so many mermaid folktales in England. This is a very old goddess, and the way they worshipped her was very crude, with drunkenness, orgies and flagellation (hitting themselves with sticks or whips). The modern, sexy image of the mermaid may be a holdover from those times. The Greeks called her Derketo, though Derketo was often her daughter. There is legend that Derketo had a love-child and out of shame she threw herself into a lake and was transformed into a fish with a woman's head.
I do not know much about Atargatis, but she is plainly a great mother goddess, a goddess of fertility. This is a type of goddess that was common in the Mediterranean. Since she is a sea goddess she symbolized the fertility of the sea. Nature renews itself continually, something that we count upon to this day. Her son Ichthys was the physical form of that fertility. In other words, he was the fish that men got from the sea. In those days, they would see the supply of fish ebb and flow and hence considered it self-renewing. This was symbolized in religion by the fact that Ichthys died and was resurrected. This event was celebrated regularly, just as the fish would appear and disappear over time.
It is interesting that Atargatis was identified with Venus. In fact, Venus was called the Syrian goddess. There is a myth that when she, along with the rest of the gods, was frightened by Typhon, she ran away to Egypt and changed herself into a fish to hid from him. Venus eventually was associated with the Virgin Mary. Her son is represented by the same fish symbol as Ichthys.